This disclosure is directed to a formation testing tool and particularly highlights certain methods of operations thereof. After an oil well has been partly drilled and has passed through formations which are thought to be producing formations, one of the next steps in the completion procedure of the well is to perform various and sundry test on formations penetrated by the oil well. One of the test techniques is to lower a formation testing tool into the oil well. Tests can then be performed for the purpose of making certain measurements (e.g. formation pressure) of interest relating to the formation. An exemplary formation testing tool is described in U.S. Pat. No. 4,375,164 assigned to the assignee of the present disclosure. As described in that particular disclosure, the tool is adapted to be lowered into the well borehole, supported on the armored logging cable which includes several conductors for providing power to the tool and surface control of the logging tool. The logging cable extends to the surface where it passes over a sheave and is stored by spooling onto a reel or drum. The conductors in the armored logging cable connect from surface control apparatus and power supplies. They also connect to a surface recording system.
One procedure known heretofore is to lower the formation testing tool a specified depth in the well. At that depth, a backup shoe is extended on one side of the formation tester and formation testing apparatus is extended diametrically opposite the backup shoe. The formation testing equipment includes a snorkel system. Primarily, this involves a surrounding elastomeric sealing pad which isolates an extendable snorkel which penetrates the formation to a specified depth. The snorkel is isolated from fluid and pressure in the well borehole to be able to test the formation only. That is, testing of the formation is conducted while isolating the formation tester from fluids and pressures in the well borehole. When the snorkel is extended into the formation, this enables direct fluid communication from the formation into the tool. This permits taking of a sample, and it isolates the sample from invasion of pressure in the well borehole. This permits a sample to be taken free of contamination of other fluids, and it permits pressure tests to be made by means of a pressure sensor to thereby obtain an accurate readout of formation pressure without distorting the data.
It has been found desirable to run a pretest, a procedure known heretofore. A pretest is implemented after a sealing pad has isolated the formation from the well borehole fluids and the snorkle has penetrated into the formation of interest. In part, the pretest is used to determine whether or not the snorkle has been properly sealed with the surrounding sealing pad, and it is also used to measure the original or beginning pressure at the snorkel in the formation undergoing test. It is possible to obtain formation pressure drawdown and buildup during the pretest sequence which aids in measuring formation permeability. This enables preliminary data to be obtained which is very useful in evaluating the particular formation. Another use is to drawdown sufficient fluid to reduce or overcome formation invasion by drilling fluid.
The present apparatus is directed to a formation tester which has the capacity of obtaining both a pretest and post test sequence, typically formation pressure drawback and buildup sequences. The post test pressure drawdown permits evaluation of formation pressure recovery. Post test data is addition information significant in evaluating the formation.
An important procedure in execution of such test is to have the capacity of extending and retracting the snorkle on command. The snorkel is routinely constructed with a filter screen on the snorkle which may become clogged or plugged at any time in the operation. Retraction and extension after retraction of the snorkel is an important feature to enable the screen area on the snorkel to be wiped clear. When this can be done, this assures additional tests can thereafter be run without distorting the data as a result of clogging the screen on the snorkel.
With the foregoing in view, the present apparatus is described as an improved formation testing apparatus capable of execution of certain improved procedures. One of the enhancement methods of operation is the post test formation pressure drawdown and formation pressure buildup sequence wherein post test formation data can be obtained. Another important procedural advantage of the present invention is the ability to periodically retract and extend the snorkel to thereby wipe the screen on the snorkel clean to prevent clogging. More will be noted concerning these and other features of the disclosed apparatus and method of use hereinafter.